An H-bridge circuit is powered by a single supply and is generally used to drive a motor or another load with bidirectional operation. FIG. 1 schematically illustrates an H-bridge circuit. As shown in FIG. 1, the H-bridge circuit comprises a first switch pair (M1 and M4) and a second switch pair (M2 and M3). When the first switch pair is turned ON, and the second switch pair is turned OFF, and the voltage at the terminal A is higher than the voltage at the terminal B, a positive current is flowing through the load with a direction (e.g. clockwise). When the second switch pair is turned ON, and the first switch pair is turned OFF, then the voltage at the terminal B is higher than the voltage at the terminal A, a negative current is flowing through the load with the opposite direction (e.g. anticlockwise).
The H-bridge circuit typically requires a load current to be sensed in some way, and the sensed load current is provided to a controller of the H-bridge circuit. Conventionally, the load current is sensed in a way that connects a sensing resistor RSENSE in series in the H-bridge, thereby the voltage across the sensing resistor RSENSE is used to indicate the load current iLOAD.
FIGS. 2-4 schematically illustrate current sensing circuits that are used to sense the load current of H-bridge circuit, respectively. A low side current sensing circuit is shown in FIG. 2, a sensing resistor RSENSE is coupled between a load and a reference ground. A high side current sensing circuit is shown in FIG. 3, a sensing resistor RSENSE is coupled between a load and an input supply VIN. FIG. 5 schematically illustrates waveform of the current sensing circuit shown in FIG. 3. Since the load current flowing upon the sensing resistor RSENSE are in opposite directions, a voltage across the sensing resistor RSENSE are discontinuous AC pulses. It would be difficult and complex to process the sensed result for a following work.
In prior art, a current sensing circuit shown in FIG. 4 is used to sense a bidirectional load current. As shown in FIG. 4, a sensing resistor RSENSE is coupled in series with a load. A differential amplifier AMP is used to amplify a voltage across the sensing resistor RSENSE. However, this differential sensing way that introduces a significant amount of common mode noise into the sensing result, thereby producing a low signal-to-noise ratio (SNR) which is undesirable.
Thus, it would be advantage to provide a current sensing circuit which is capable of sensing a bidirectional current with a much higher SNR than conventional circuits.